The vast majority of satellite navigation applications are currently based on the Global Positioning System (GPS) controlled by the United States Departments of Defense and Transportation. This scenario will significantly change with the advent of GALILEO.
GALILEO is a European initiative for a global navigation satellite system (GNSS), providing a highly accurate global positioning service under civilian control. While providing autonomous navigation and positioning services, GALILEO will be interoperable with GPS and GLONASS, another global satellite navigation system. A user will be able to take a position with the same receiver from any of the satellites in any combination. By offering dual frequencies as standard, however, GALILEO may deliver real-time positioning accuracy down to the metre range. It will strive to guarantee availability of the service under all but the most extreme circumstances and will inform users within seconds of a failure of any satellite. This will make it suitable for applications where safety is crucial, such as running trains, guiding cars and landing aircraft. The combined use of GALILEO and other GNSS systems may offer much improved performance for all kinds of user communities.
In the new generation of GNSSs, attention has been given to have efficient and spectrally relevant signals. GALILEO and GPS will share two central frequencies and will both send several signals on the same carriers. Consequently, new signal modulations had to be studied to minimize inter- and intra-system interference. One modulation emerged due to its split spectrum that spectrally isolates the signal from the currently used Bi-Phased Shift Keying (BPSK) modulation [Godet et al., 2002; Betz, 2002]. This new modulation is known as Binary Offset Carrier (BOC). The BOC modulation is part of the GALILEO signal plan.
As used herein, “BOC” refers to a signal resulting from a modulation which multiplies a pseudo-random noise (PRN) spreading code with a square wave sub-carrier (SC) that has a frequency multiple of the code rate. It creates a symmetric split spectrum with two main lobes shifted from the center frequency by the frequency of the sub-carrier. The properties of a BOC signal are dependent on the spreading code chip rate, the sub-carrier frequency, and the sub-carrier phasing within one PRN code chip. The common notation for BOC-modulated signals in the GNSS field is BOC(fc,fs) where fc represents the code chip rate, and fs is the frequency of the sub-carrier. Both fc and fs are usually noted as a multiple of the reference frequency 1.023 MHz. BOC(n,m) may then be expressed as PRNm*fc×SCn*fc.
A BOC signal induces better tracking in white noise and better inherent multipath mitigation compared to the spreading code alone. However, it also makes acquisition more challenging and tracking potentially ambiguous due to its multiple peak autocorrelation function. A summary of the basic properties and improvements brought by BOC signals compared to BPSK signals is given by Betz (2002).
As already mentioned, the presence of a sub-carrier in the BOC signal introduces secondary peaks in the range [−1, +1] chip in BOC autocorrelation. The presence of these secondary peaks may cause a serious problem if the receiver locks onto a side peak instead of the main peak. A significant bias of approximately 150 m would then be present in the range measurements, which is unacceptable for navigation applications.
Several methods have been proposed to track BOC signals without suffering from any potential tracking bias. Fine and Wilson (1999), Lin et al. (2003), Martin et al. (2003) and Ward (2004) are a few examples. They treat the problem of the BOC tracking ambiguity in a broad sense, trying to find a solution that could be applied to any BOC(n,m) signal. Each of these suffers from various disadvantages.
Therefore, there is a need in the art for efficient methods of acquiring and tracking a BOC signal which minimizes potential tracking bias and allows unambiguous tracking of the signal.